A well-known problem with strain gauge pressure sensors connected as a Wheatstone bridge is that of the zero point of the bridge drifting as a function of temperature.
To resolve that problem, use is generally made of compensation networks connected to the inputs and/or the outputs of the bridge. Those networks which include at least one temperature-sensitive element, are generally designed to provide linear compensation, given that the temperature drift of strain gauge bridges is substantially linear over ordinary temperature ranges.
An additional difficulty arises when the pressure sensors are used at very low temperatures. The temperature drift of strain gauge bridges then becomes non-linear, and the non-linearity increases with decreasing temperature. This can be seen from FIG. 1 which shows how the output voltage from a strain gauge bridge made up of thin film nickel-chromium deposits on a silicon substrate varies as a function of temperature, the bridge being at equilibrium (zero output voltage) at a temperature of 22.degree. C.
Ordinary compensation networks become ineffective, particularly since the thermistors used generally also have resistance that becomes very high and virtually infinite when the temperature drops below -40.degree. C. or -50.degree. C.
The problem which the present invention seeks to resolve is that of compensating the non-linearity of temperature drift, and consequently that of "linearizing" such drift at very low temperatures, in particular at temperatures below the boiling point of nitrogen, i.e. below about -196.degree. C.